Diagnostic advances in the field of medical imaging coupled with technological progress in digital storage media, digital processing of video, as well as audio data and Very Large Scale Integration (VLSI) device manufacturing, are converging to make the display, archiving, and transmission of digital video economical in a wide variety of medical applications.
Buffering, storage, and transmission of digitized video is a desired and critical feature for most medical applications. Because uncompressed digital video requires large amounts of bandwidth, memory, and storage space, video image compression is vital in the design of economically feasible display and archiving systems.
Recent studies of compressed cardiac ultrasonic and X-ray still images suggest that compression techniques have the potential to offer a compromise between traditional image recording techniques and economics of real-time diagnostic video compression. The advances in lossless and lossy video compression algorithms will further improve the quality of compressed video in the future.
Unlike in ordinary video signals, medical applications require a precise correlation of every video frame in a video sequence to physiological events occurring in the imaged anatomies or processes, and further require an adaptive adjustment of the frame rate to suit these display requirements. It is therefore diagnostically advantageous to think of a diagnostic video as a sequence of digital video events, each corresponding to a physiological phenomenon such as a heart contraction or relaxation.
For example, the practice of ultrasonography requires video recording of patient data for diagnostic and record keeping purposes. A typical ultrasonographic study comprise a small number of still images and few minutes of full motion video recording. Also, in special applications, the simultaneous display of multiple cardiac cycles, such that side-by-side comparisons of previously recorded video sequences can be made with live video sequences, is required during an examination for diagnostic purposes.
Often times, access to patient records in the form of digital motion video is required so that physicians of various specialties located in different areas can participate in the diagnostic or review process concurrently.
Although most diagnostic imaging systems provide some sort of cine' loop review, they typically do not provide digital motion video recording, serial comparison, and display functions. Typically, the video recording is accomplished by professional grade video tape recorders using Super-VHS (S-VHS) format tapes attached to the diagnostic imaging system (DIS).
Diagnostic video is recorded at a constant frame rate (FR) (typically 30 frames per second or fps) onto the video tape, and bears no relevance to the physiological function of the imaged anatomy or process, such as cardiac cycles. A tedious manual process is required to search through the video tape to identify the appropriate video frames corresponding to the systole or diastole. The video taped echocardiographic examination data, such as patient name, identification number, machine settings, etc., are available only in a visual format. In other words, this information cannot be used for databasing applications.
On the whole, the diagnostic video review process is very time consuming and difficult due to the operational limitations of the video cassette recorder. Copies of originally recorded diagnostic video result in image resolution degradation which renders them more difficult to interpret accurately. Also, video tapes require a lot of storage space, and degrade further with time. Thus, video tapes are not an ideal media for medical record keeping.
In another example, angiographic coronary arteriography and ventriculography studies are performed either to diagnose the presence of heart disease or to aid in a procedure called Percutaneous Transluminal Coronary Angioplasty (PTCA).
Although digital motion video capture and visualization systems for X-ray angiography do exist, they are significantly different from the system to be described herein. The present day digital angiography systems do not utilize real-time video data compression, nor do they store routinely data from the completed studies to a removable mass storage media. Rather, the images are stored to a large random access memory (RAM) based image buffer for instant replay and visualization. Only selected still images of arteriograms and venriculograms are archived to an optical disk. The size of the buffer varies, but rarely exceeds 120 seconds worth of full motion video. The images are also simultaneously recorded on 35 mm cine' film for archiving. The digital angiography motion image review systems usually allow for repetitive visualization of single contrast injections, but do not allow for visual serial comparisons of selected cardiac cycles from different contrast injections.
More recent X-ray imaging equipment currently utilizes charge coupled devices (CCD) in place of traditional fluoroscopy, with full digital image archiving of uncompressed digital motion video to a high bandwidth video tape recorder. However, the high cost of such recorders limit their application to playback on dedicated workstations only, and does not allow for digital video image transmission, serial comparisons, and display in a multiple window fashion. The availability of multiple cardiac cycle display would be particularly useful during a PTCA procedure so that the progress of plaque removal from the arteries (called revascularization) can be monitored.
Furthermore, in the standard practice of coronary angiography, a 35 mm cine' film recording is always made, regardless of any digital image storage capability, due to the need for inexpensive means of image review and overreading by a referring physician. Thus, since the patient examination data is typically available only in the cine' film and the digital media format, it is impossible or very difficult to integrate all of the pertinent data into digital transmission networks.